Just had an email come from TfL with the weekend travel update and this was in it:
Due to a lack of available trains, there will be a reduced service between Gospel Oak and Barking. Trains will run every 15 to 30 minutes.
This is because of a delayed delivery of new electric trains from the manufacturer, Bombardier Transport, as part of our £300m investment to boost capacity on the busiest Overground lines. We continue to operate diesel trains on this line, but they are becoming increasingly unreliable and are now in need of additional maintenance.
The reduced weekend timetable will continue until the new electric trains are introduced. We will continue to run a full timetable on the busier weekday service.